It has been known for some time that aerosolized formulations of drugs can be inhaled and absorbed through the lungs. The formulation can treat lung tissue topically and/or be absorbed into the circulatory system to have a systemic effect. In general, aerosolized particles must have a diameter of 12 microns or less. Topical lung treatment can be carried out using particles having a diameter in the range of 8.0 to 12.0 microns. However, these particles are generally too large to carry out affective systemic treatment which generally uses particles having a diameter in the range of 0.5 to 6.0 microns.
It is difficult to produce particles which are sufficiently small inhalation i.e., particles having a diameter of less than 12 microns. There are several different types of devices which allow for the production of these small particles. In one method referred to as a "dry powder inhaler" or DPI the particles are dispensed by means of bursts of large volumes of compressed air which entrain small amounts of the particulate to form a dust cloud or atomize some of a fluid to form a spray of fine droplets. However, this method results in losses of medicament and requires that the user have a source of large volumes of compressed air available. A more commonly used device is a "metered dose inhaler" or MDI which uses a low boiling point propellant in a pressurized canister. By releasing a valve on the canister a metered amount of formulation and propellant are released. The propellant "flashes" or quickly evaporates away leaving small particles of drug for inhalation. Although an MDI provides a self-contained and readily portable device to be constructed, the use of liquefied propellants is increasingly unacceptable from environmental and other grounds.